Time for some 'high speed' action! The letters are sent whizzing through a postmarking machine that prints an ink mark over your stamp. This postmark tells you which Centre processed your letter,at what time and on what date. The postmark also 'cancels' your stamp so no-one can use it again. Oops!... there's a large envelope that's too big to go through the machine and there's another with the stamp in the wrong place. They'll need to be postmarked by hand.